Preferential 〈220〉 crystalline growth in nanocrystalline silicon films from 27.12 MHz SiH₄ plasma for applications in solar cells

Abstract

It has been experimentally demonstrated that silicon nanocrystallites (Si-ncs) are generally of 〈111〉 crystallographic orientation from random nucleation, which are associated to highly defective polyhydride networks at the grain-boundary; however, ultra-nanocrystallites preferably harvest a 〈220〉 alignment due to the thermodynamically preferred grain growth with concomitant monohydride bonding at the boundary. Using an excitation frequency (27.12 MHz) higher than the conventional frequency of 13.56 MHz, and its stimulus impact in terms of larger ion flux densities with reduced peak ion-energy in the plasma and its associated ability to efficiently generate atomic hydrogen, nanocrystalline silicon (nc-Si) films are produced. The nc-Si:H films grown at elevated pressures demonstrate enhanced growth rates, lower hydrogen contents, lower microstructure factors, preferred 〈220〉 crystallographic orientation and possess a significant fraction of ultra-nanocrystalline component in the Si-network, along with a higher intensity of monohydride bonding at the grain boundary by bond-centered Si–H–Si modes in a platelet-like configuration. The material prepared at a low power and low temperature is extremely suitable, in every aspect, for efficient application in the fabrication of nc-Si p–i–n solar cells.

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Introduction

Hydrogenated nanocrystalline silicon (nc-Si:H) deposited by plasma enhanced CVD (PECVD) is a much-admired material for thin film devices, e.g., solar cells and thin film transistors (TFT).^1–7 Over the past few decades, capacitively coupled systems (CC-PECVD) powered by radiofrequencies (RF) have been widely used in thin film industries due to its associated high deposition rates and good material qualities.^8–10 For further enhancing the growth rate and improving the quality, very high-frequency (VHF) sources are now being used.^11–14 In view of its successful integration in multilayered device structures, and to make various substrates usable for the manufacturing of devices, lowering the deposition temperature for the growth of a device grade nc-Si:H network is one of the pressing agendas in the use of high frequency plasmas. Regarding material issues, it has been identified that the opto-electronic devices made from films with Si-ncs having dominant 〈220〉 crystallographic orientation demonstrate better performances because of the relatively improved electronic properties of such materials. In particular, the best open-circuit voltage (V_OC) values in silicon solar cells identifies low recombination losses in 〈220〉 textures.^15–17

The present work deals with exploring the influence of the higher excitation frequency of 27.12 MHz, compared to the conventional frequency of 13.56 MHz, in preferential harvesting of the thermodynamically superior 〈220〉 aligned crystallographic planes in nc-Si growth, even at a low substrate temperature. This is accomplished by utilizing its associated larger ion flux densities along with the reduced peak ion-energy in the plasma and its corresponding ability to efficiently generate atomic hydrogen. Spectroscopic analysis has been performed for identifying specific hydrogen bonding configurations in relation to the growth of silicon ultra-nanocrystallites (unc-Si) and the 〈220〉 crystallographic orientation.

Experimental

The nc-Si:H films of thickness ∼400 nm were prepared by a conventional capacitively coupled RF plasma reactor with an excitation frequency of 27.12 MHz while keeping the hydrogen dilution (H₂/SiH₄) fixed at 98%. Films were deposited on Corning® Eagle2000™ glass and silicon 〈100〉 wafer substrates, and the gas pressure was optimized within 1–8 Torr, while the substrate temperature (T_s) was fixed at 180 °C and the RF power applied between the electrodes was kept constant at 40 W. The deposition system with a load-lock chamber was maintained at an ultra-high vacuum, in the order of 10⁻⁷ Torr; this provided an almost contamination-free environment for film deposition. The samples were characterized by X-ray diffraction analysis, which was carried out using a conventional Cu-K_α X-ray radiation (λ = ∼1.5418 Å) source and a Bragg diffraction setup (Seifert 3000P). Raman spectra of the samples were obtained from a Renishaw in Via Raman spectroscope with an excitation wavelength of 514 nm from an air-cooled Ar⁺ laser source at a power density of ∼2 mW cm⁻². The silicon hydrogen bonding structure was investigated using a Fourier transform infrared (FTIR) spectrophotometer (Nicolet Magna-IR 750) with films deposited on the polished surface of single crystal Si wafers.

Results

Deposition rate

Fig. 1 shows the variation of the film deposition rate (R_d) that increases, in general, with pressure from 9 nm min⁻¹ at p = 1 Torr to 35 nm min⁻¹ at p = 8 Torr. Careful observation identifies two distinct natures of variation for R_d. The deposition rate increased with almost similar slopes in the low pressure (1–3 Torr) and high pressure (6–8 Torr) regions; however, R_d increased with a considerably high slope in the medium pressure region, around 3 < p < 6 (Torr), signifying that a radically different growth process is in effect.

Fig. 1 Variation of the deposition rate of nanocrystalline silicon thin films on glass substrates at different gas pressures (p).

X-ray diffraction

The crystallinity in the Si:H network was investigated by X-ray diffraction studies. The XRD spectra of nc-Si:H films, prepared at different deposition pressures, shown in Fig. 2, exhibit three dominant peaks corresponding to the 〈111〉, 〈220〉 and 〈311〉 crystallographic orientations of c-Si, identified at 2θ = 28.3°, 47.2° and 56.2°, respectively. It was previously suggested that the growth along the 〈220〉 direction is thermodynamically preferred and the grains with 〈220〉 planes are formed at an elevated temperature and high pressure, while the 〈111〉 peak arises due to random nucleation.^15,18,19 To understand the proportional strength of the specific crystallographic alignment of the nc-Si:H network, the orientation factor (Q) is defined as:

Q = I_〈220〉/I_〈111〉,

where I_〈111〉 and I_〈220〉 represent the intensities of the 〈111〉 and 〈220〉 diffraction peaks, respectively. In the case of silicon powder, the normalized peak intensities of the 〈111〉, 〈220〉 and 〈311〉 directions hold as the specific ratio of, I_〈111〉 : I_〈220〉 : I_〈311〉 = 100 : 55 : 30, according to the database of powder diffraction files JCPDS-[27-1402]. Therefore, Q = 0.55 is a critical value of the orientation factor to determine the preferred 〈220〉 crystallographic alignment of the nc-Si:H network. The ratio of I_〈220〉/I_〈111〉 is evaluated for this set of films and is shown as the inset of Fig. 2, which demonstrates that I_〈220〉/I_〈111〉 systematically increases with increasing gas pressure and attains the maximum magnitude of ∼0.97, i.e., both the components contribute nearly equally towards crystallinity at a gas pressure of 7 Torr.

Fig. 2 XRD spectra of the silicon films prepared at different pressures. The inset represents the change in the ratio of 〈220〉 to 〈111〉 peak intensities, I_〈220〉/I_〈111〉, with increasing pressure.

The ratio, however, decreases on further increasing the pressure, attaining ∼0.89 at 8 Torr. The preferential growth of the nc-Si grains along the 〈220〉 longitudinal direction facilitates the charge carriers to conduct perpendicular to the substrate in stacked layer devices, e.g., in the nc-Si solar cells, and interact with fewer grain boundaries than they do for the 〈111〉 oriented grains, while the a-Si:H component further helps to efficiently passivate the grain surface. The combination of these two phenomena serves to reduce bulk recombination and field losses and thereby, in general, increases the open-circuit voltage (V_OC) and fill factor (FF) of the nc-Si solar cell.^15–17 In the present investigation, nc-Si:H films having the preferred 〈220〉 orientation of strength, Q = ∼0.97 is formed at p = 7 Torr at a low substrate temperature (180 °C) compatible to the fabrication of solar cells.

The average grain size (D) of the nanocrystallites has been estimated from the FWHM (β) of each diffraction peak in the XRD spectra, using Scherrer's formula:

D = 0.9λ/β cos θ.

It shows a close resemblance in its nature of variation with increasing pressure along different crystallographic orientations, as shown in Fig. 3. A maximum grain size of the Si-ncs ∼9.5 nm along the 〈111〉 direction and ∼11 nm along the 〈220〉 direction has been obtained.

Fig. 3 The variation in grain size of silicon nanocrystals with pressure.

Raman spectroscopy

The Raman spectra for the films prepared at different gas pressures have been presented in Fig. 4. Each Raman spectrum can be deconvoluted into three Gaussian components; the component on the higher frequency side (at around 515 cm⁻¹) represents the nanocrystalline part of the material and the lower frequency component (at ∼480 cm⁻¹) arises from the amorphous part, whereas the intermediate one (at ≤ 510 cm⁻¹) corresponds to the ultra-nanocrystalline and/or the grain boundary component. The typical deconvoluted satellite components for the film prepared at p = 7 Torr has been included in Fig. 4. Careful observation of the magnified view of the Raman peaks, shown at the inset in Fig. 4, clearly demonstrates a gross shift in the peak positions, which is a consequence of the changes in the average size of the nanocrystallites in the network as an effect of changes in the gas pressure during growth.

Fig. 4 Normalized Raman spectra of silicon thin films prepared at different pressures (p). The gradual shifting of peak position is observable from the magnified view in the inset.

The gross crystalline volume fraction X_C in the films can be calculated using the following equation:^20,21

where I_i is the area under the Gaussian centered at i and (I₅₂₀ + I₅₁₀ + I₄₈₀) is the total integrated intensity. After deconvolution of each individual spectrum, the gross crystalline volume fractions were estimated and are shown in Fig. 5, which reveal that the X_C increases systematically from 72% at 1 Torr to 87% at 4 Torr and then starts gradually decreasing with further increase in pressure, attaining a magnitude of 74% at 8 Torr.

Fig. 5 The crystalline volume fraction (as estimated from Raman spectroscopy) and the variations of the mean size of Si film at different pressures. Inset shows the variation of the UNC/NC ratio with pressure.

The average grain size has been estimated from first order Raman spectra using the following equation.^22,23

where ω_L is the frequency of the crystalline-like mode for a particle of size L and Γ₀ is the natural line width (inversely proportional to the intrinsic phonon lifetime). In the case of crystalline silicon, the values of ω₀ and Γ₀ are 520 and 3.5 cm⁻¹ respectively. The nanocrystallites attain a maximum size of ∼10.6 nm at an optimum pressure of 4 Torr, while the size of the nanocrystals is miniaturized on both sides of the pressure variation. The nature of size variation for the nanocrystals resembles the variation of gross crystalline volume fraction in the Si-network, X_C, as shown in Fig. 5.

It is interesting to note that the relative population of the ultra-nanocrystalline fraction (X_unc) to the nano-crystalline component (X_nc) has been found to increase monotonically with increasing gas pressure up to p = 7 Torr, above which it reduced in magnitude, as shown in the inset of Fig. 5. Accordingly, it is demonstrated that during increases in the gas pressure in the plasma, the material attains the maximum crystallinity corresponding to possessing the largest grain-size of the nanocrystallites simultaneously with a high magnitude of X_unc/X_nc at p = 4 Torr. However, further increasing the pressure reduces the gross crystalline volume fraction (X_C), simultaneously lowering the size of the nanocrystals, although the ultra-nanocrystalline component keeps on preferentially populating in the network to a certain extent.

FTIR study

To understand the bonding structures between silicon and hydrogen in the network, infrared absorption studies have been performed with samples on Si-wafers. Fig. 6 shows the typical absorption coefficient spectra of the films prepared at different pressures in the region around 550–750 cm⁻¹. In the case of an amorphous silicon network, the absorption band within the range of 550–750 cm⁻¹ usually contains the exclusive contribution of Si–H wagging bonds only with its characteristic peak at around 640 cm⁻¹. However, for the present highly nanocrystalline silicon films, the band in the region of 550–750 cm⁻¹ splits into two well-resolved peaks centered at around 630 and 690 cm⁻¹, and the corresponding bands are associated to the Si–H wagging and SiH_n (n ≥ 2) rocking vibrational modes, respectively. With the increase in pressure from 1 Torr to 4 Torr, the increasing crystallization in the network could be closely correlated to the increasing relative strength of the poly-hydride component along with the reduced overall intensity of the absorption band, thus signifying reduced hydrogenation of the network. The bonded hydrogen content (C_H) in the films has been estimated from the Si–H wagging mode vibrational component of the absorption band at 630 cm⁻¹ as

C_H = (A_ω/N_Si)∫αdω/ω × 100 at.%,

where A_ω = 1.6 × 10¹⁹ cm⁻² is the corresponding oscillator strength and N_Si = 5 × 10²² cm⁻³ is the atomic density of crystalline silicon.^24,25

Fig. 6 Wagging mode IR absorption spectrum in the wavenumber range 550–750 cm⁻¹.

A variation of bonded hydrogen content from 10% to 4% with increase in pressure from 1 Torr to 8 Torr has been shown in Fig. 7. It is evident from the figure that C_H changes only marginally for an initial increase in p from 1 to 3 Torr, beyond which C_H continuously decreases with the increase in p. However, a relatively sharp reduction in C_H has been markedly noted for a change in p from 3 to 4 Torr, corresponding to the attainment of the highest crystallinity in the network.

Fig. 7 Variations in the bonded hydrogen content and the microstructure factor of the nc-Si films prepared at different pressures.

The absorption coefficient in the range of 810–925 cm⁻¹, shown in Fig. 8, consists of two separate bands at 850 and 900 cm⁻¹, characteristic of bending vibrational modes of poly-hydride (Si–H₂)_n complexes (isolated or coupled) and di-hydride Si–H₂, respectively.²⁶ It can be inferred from the result that, in the course of reduced hydrogenation in the network at higher pressure, the isolated di-hydrides of silicon, Si–H₂, are preferentially accumulated in the form of clusters of (Si–H₂)_n.

Fig. 8 IR absorption spectrum in the wavenumber range of 810–925 cm⁻¹.

In addition, a considerable change in the shape and intensity of the absorption band at 1900–2220 cm⁻¹ has been observed, as shown in Fig. 9. Contributions to the stretching mode absorption band was attributed to (i) 1990 cm⁻¹ component demonstrating the mono-hydride SiH configurations in the bonding structure, (ii) a bond-centered hydrogen, Si–H–Si, identified as hydrides in a platelet-like configuration by the corresponding absorption band around 2040–2050 cm⁻¹, (iii) a di-hydride Si–H₂ and poly-hydride (Si–H₂)_n mode near 2100–2110 cm⁻¹, and (iv) a tri-hydride SiH₃ stretching mode between 2140 and 2150 cm⁻¹. It has been identified that the intensity of the mono-hydride (Si–H) bond and the bond-centered hydrogen in a Si–H–Si configuration increased linearly with pressure, whereas the intensity of di-hydride Si–H₂ or poly-hydride (Si–H₂)_n modes decreased with increasing pressure, and the tri-hydride configuration increased correspondingly with increased crystallinity with its maximum intensity at p = 4 Torr.

Fig. 9 Absorption coefficient spectra of deconvoluted Si–H mono-hydride and poly-hydride components and their variations with increasing pressures, indicating the formation of platelet hydride.

The microstructure factor R, defined as the fraction of poly-hydride component in the network, is presented as follows:

R = (I₂₁₀₀ + I₂₁₄₀)/(I₁₉₉₀ + I₂₀₄₀ + I₂₁₀₀ + I₂₁₄₀),

where ‘I’ represents the integrated intensity under the corresponding satellite absorption bands and has been estimated for samples prepared at different pressures. The microstructure factor R has been found to gradually decrease from 0.70 to 0.40 as pressure increased from 1 Torr to 8 Torr (Fig. 7). Reducing microstructure factor, in general, corresponds to the elimination of structural imperfections in the network, e.g., the presence of defects, voids and dangling bonds. Although the sample prepared at 4 Torr has the highest crystallinity, its microstructure factor is less than that of the relatively less crystalline film prepared at a low pressure. This result happens to be opposite to the conventional H₂ diluted plasma result and appears to be interesting as well.

Discussion

It is argued that the influence of the excitation frequency on the plasma, higher than the conventional frequency of 13.56 MHz, plays a key role on the growth kinetics by allowing a highly effective dissociation of the process gas by virtue of the associated higher electron density and larger ion flux densities, leading to an enhanced growth rate of the material.¹¹ In addition, the peak ion-energy in the plasma reduces grossly with increasing frequency, providing a much lower ion impact energy on the growing surface of the film; this could effectively reduce the surface damage and lead to superior material quality. Moreover, a sufficient amount of low-energy ion bombardment happens to be beneficial for the growth, as it may increase the surface mobility of the radicals and desorption of reactive species.²⁷ The advantages of the lower electron temperature involved in the higher excitation frequency of the plasma and its ability to more efficiently generate atomic hydrogen^13,28 could be beneficial to promote the growth of nanocrystalline silicon at considerably lower levels of power and temperature as compared with the standard glow discharge at 13.56 MHz.^29,30

Presently, the deposition rate of the nc-Si thin films only varies from 9 to 35 nm min⁻¹ with increasing gas pressure p from 1 to 8 Torr. This relatively low deposition rate is due to a very low magnitude of applied electrical power (40 W); however, by virtue of the relatively high frequency (27.12 MHz) of the electrical field (compared to the conventional frequency of 13.56 MHz) and its associated favorable interactions, films of ∼87% crystalline volume fraction with Si-ncs of average size ∼10 nm having the preferred 〈220〉 crystallographic orientation has been obtained at a substrate temperature as low as ∼180 °C. This is compatible with the fabrication of solar cells. Films produced at higher pressures have, in general, lesser hydrogen content, lower microstructure factor, are dominantly 〈220〉-oriented and possess a significant fraction of ultra-nanocrystalline component.

It is interesting to carefully monitor the evolution of the stretching mode of silicon–hydrogen bonding at around 2040 cm⁻¹ that has been attributed to the hydrides in a platelet-like configuration and is not commonly observable in the case of nanocrystalline silicon. The hydrogen induced platelet formation in crystalline silicon (c-Si) has been reported earlier,³¹ along with its theoretical investigations for different structural models.^32–35 However, very few investigations have been done on the platelet formation in nc-Si material during its growth by PECVD. It was earlier reported that the peak arises due to the formation of a hydrogen-dense compact grain boundary structure with good passivation in nc-Si:H thin films.³⁶ According to Xu et al.,³⁷ the process of H induced crystallization of silicon thin films by the insertion of H into the stressed Si–Si bonds leads to the formation of (a) a bond-centered hydrogen, Si–H–Si (b) an isolated silicon, H–Si– (dangling bond) and (c) a Si–Si bond with a bond length close to the equilibrium c-Si bond length, where the H atom is bonded to only one Si atom. The H inclusion and diffusion of H are the main reason for forming platelet-like hydrides within the nc-Si:H films.^38,39 At elevated pressures, the insertion of H into the network takes place at a higher intensity of the mono-hydride mode accompanied by its platelet-like configuration, as observed in Fig. 9. The inclusion of H atoms into strained Si–Si bonds and its diffusion through the formation of intermediate bond-centered Si–H–Si configurations produces platelet-like hydrides.⁴⁰ The H-insertion reactions into the bond-centered location facilitate the reorientation of the Si network from disorder-to-order transformation. In particular, bond breaking and reforming reactions are facilitated by H addition, which results in the elimination of strained Si–Si bonds and concerted atomic rearrangements. Thus, the platelet-like hydrides help to grow an ordered and compact film even at higher pressures.

Evolution of the 2050 cm⁻¹ band, which is characteristic of polymorphous silicon films, corresponds to an elevated flux of clusters to the substrate before the onset of powder formation.⁴¹ The formation of enhanced platelet hydrides leads to increased deposition rates for the films from typically 9 nm min⁻¹ to 36 nm min⁻¹ with increased pressure from 1 Torr to 8 Torr along with a simultaneous improvement in the order of the material,⁴² particularly in terms of increasing preferential growth towards a 〈220〉 crystallographic orientation, which may allow increases to the efficiency and the stability of p–i–n solar cells.⁴³

The explicit structural morphology of the films is determined by the crystalline volume fraction, the grain size and the specific crystallographic orientations, which altogether are governed by the nature of the growth precursors involved in the formation of the network on the substrate after their generation in the plasma and the interactions with the energetic species, e.g., atomic-H, excited states of Ar (Ar*) or He (He*) and ions of the plasma, with the growing surface of the solid silicon matrix.^5,44,45 It is believed that Si–H₃ is the main precursor responsible for the film growth in a silane–hydrogen gas mixture, while other lower hydride precursors like Si–H_x (1 ≤ x ≤ 2) and ions play a crucial role to control the properties of the material, as they are more active. Comparing the gas phase diffusion length of Si–H₃ and Si–H_x (1 ≤ x ≤ 2), it is accepted that the lifetime of Si–H_x (1 ≤ x ≤ 2) is shorter than Si–H₃ and this shorter lifetime conveys their reduced diffusion length at the absorption surface on the growing network. At an elevated gas pressure, the number density of lower hydrides of silicon and more reactive SiH_x (x < 3) radicals and ions increases in the plasma, which in general helps increasing the growth rate of the material. However, a critical balance of available SiH₃ precursors to its lower hydride components and an efficient out diffusion of bonded H from the network promotes the highest crystallinity with the largest grain size. On further increasing the pressure, lowering in crystallinity with reduced grain size are the obvious consequences of increasing growth rate, while the subsequent continuous lowering in the microstructure factor could be assigned to the increasing ultra-nanocrystalline fraction (UNC/NC) of overall crystallinity in the network, which is markedly different from the usual observations available in the literature. The significance of ultra-nanocrystallinity has been further probed by monitoring its relevance in controlling the orientation factor Q = I_〈220〉/I_〈111〉, demonstrated by the one-to-one correspondence with their preferential growth, as shown in Fig. 10.

Fig. 10 Variation of I_〈220〉I_〈111〉 with UNC/NC ratio of the silicon film prepared at different pressures.

From the experimental evidence, it could be inferred that the nano-crystallites, in general, are of 〈111〉 crystallographic orientation and are associated to highly defective poly-hydride networks at the grain-boundary, while the ultra-nanocrystallites preferably harvest a 〈220〉 alignment with concomitant mono-hydride bonding, that is, a bond-centered hydrogen in a platelet-like configuration at the boundary. The 〈111〉 orientation arises from random nucleation, whereas 〈220〉 is due to the thermodynamically preferred grain growth with the lowest surface energy at this crystal plane.⁴⁶ In addition, crystalline grains with a 〈220〉 crystallographic orientation are conducive to carrier transport and are thus preferably useful in devices.^47,48

Accordingly, the formation of the enhanced ultra-nanocrystalline component in the Si-network at elevated pressure, under the optimized parametric condition in 27.12 MHz H₂-diluted SiH₄ plasma, promote nc-Si films of low microstructure factor with sufficiently high crystallinity (≤80%), small grain size (≤6 nm) and the preferred 〈220〉 crystallographic orientation at a low substrate temperature and an enhanced growth rate, which, in every aspect, are favorable for the efficient application of the material in the fabrication of nanocrystalline silicon solar cells.

Conclusion

Nanocrystalline silicon (nc-Si) films of ∼87% crystalline volume fraction with Si-ncs of an average size of ∼10 nm and preferred 〈220〉 crystallographic orientation have been obtained at a low electrical power (∼40 W) with a substrate temperature as low as ∼180 °C under the optimized parametric condition of 27.12 MHz in an H₂-diluted SiH₄ plasma. In general, the nc-Si films produced at an elevated pressure have enhanced growth rates, lesser hydrogen content, lower microstructure factors, a preferred 〈220〉 crystallographic orientation and possess a significant fraction of ultra-nanocrystalline components in the network. At an elevated pressure, the insertion of H into the network takes place at a higher intensity of mono-hydride bonding accompanied by its bond-centered Si–H–Si mode in a platelet-like configuration.

Experimental evidence infer that the nano-crystallites are, in general, of 〈111〉 crystallographic orientation from random nucleation and are associated to highly defective poly-hydride networks at the grain-boundary, while the ultra-nanocrystallites preferably harvest a 〈220〉 alignment due to the thermodynamically preferred grain growth with concomitant mono-hydride bonding, that is, bond-centered hydrogen in a platelet-like configuration at the boundary.

Influence of the higher excitation frequency 27.12 MHz, compared to the conventional frequency of 13.56 MHz, in terms of larger ion flux densities with the reduced peak ion-energy in the plasma and its associated ability to effectively generate atomic hydrogen help in producing nc-Si films having the abovementioned characteristics, which, in every aspect, are favorable for the efficient application of the material in the fabrication of nanocrystalline silicon solar cells.

Acknowledgements

The work has been done under nano-silicon projects funded by the Department of Science and Technology (Nano-Mission Program) and Council of Scientific and Industrial Research, Government of India.

Notes and references

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